Good thermal design extends LED lifetimes

Guest columnist Adrian Rawlinson, managing director of Marl International describes techniques for the thermal design of high brightness LED packages in lighting systems

LEDs with a power dissipation of 35W or more are now available. Such lights are so bright that they are uncomfortable, if not dangerous, to look into directly, but with increasing power, there is increased thermal load and more heat to dissipate. Adherence to the sound thermal design principles described in this article will ensure the best performance and the longest life.

LED lights are based on tiny semiconductor diodes, and it is essential that the junction temperature of these diodes is kept at or ideally well below the rated maximum to achieve the maximum service life and efficiency and to ensure it delivers the correct colour temperature.

The surface area of an LED package is quite small, and the package itself will release little heat to the atmosphere. An external radiator, such as a heat sink, is therefore required.

When designing a system, a heat sink should be selected with sufficient cooling capacity to keep the die junction below 120°C. Even within this limit, LED die junction temperatures can affect dominant and peak wavelength and cause slight shifts in colour temperature for LED white light sources. The junction temperature of the light-emitting element Tj must be kept below the absolute maximum rating in the specifications under all conditions to be encountered in service. Direct measurement of the junction temperature of a light-emitting element inside a package is seldom possible, so the temperature of a particular part on the package outer shell (the case temperature) Tc is normally measured. Tj is calculated from the thermal resistance between the junction and the case Rj-c, measured in °C/W, and the amount of emitted heat, which is nearly equal to the input power Pd.

A key point is that the light-emitting element is not mounted on the insulating layer, which has low thermal conductivity, but directly on the aluminium substrate. In this way, the heat generated at the light-emitting element can be efficiently conducted to the outside of the package.

The aluminium substrate side of the package outer shell thermally connects to the heat sink via heat-dissipative grease (or adhesive). The heat generated in the junction section of the light-emitting element is thus transferred as conductive heat via the element-mount adhesive, the aluminium substrate, and grease (adhesive) to the heat sink. Thermal analysis using SolidWorks Cosmos 2007 software gives the temperature distribution and heat flowing in the product, as well as the heat exchanged between the product and its environment.

The movement of heat through the component and heat sink into the environment can be pictured by plotting heat flux vectors. The investment in software is amply repaid by the results it achieves. All prototypes are subjected to tests in a temperature chamber, and the measured temperatures are normally within 3-4°C of the simulation result. A final point to note is that performance characteristics of LED light sources are specified for a rated current and 25°C LED die junction temperatures.

Since most LEDs operate well above 25°C, these values should be considered for reference only and the light output should be based on the anticipated operating temperatures. Applications requiring specific wavelengths or colour temperature should take this effect into account when designing drive conditions and heat sinking.